Comprehensive Photophysical Properties of Thiophene/Phenylene Co-oligomers Investigated by Theoretical and Experimental Studies

The photophysical properties of a 2,5-bis(4'-cyanobipheny-4-yl)thiophene (BP1T-CN) molecule as a function of solvent polarity are intensively studied by means of steady-state spectra and femtosecond to nanosecond transient absorption spectroscopy combined with quantum chemical calculations. It is found that the lowest singlet excited state of BP1T-CN contains the contribution from both the locally excited (LE) and intramolecular charge transfer (ICT) components, which dominated from the LE state to the ICT state with the increase in solvent polarity. Femtosecond transient absorption experiments further confirm this evolution upon photoexcitation and provide a clear picture of the excited-state deactivation processes within the 415-780 nm spectral region, where the excited-state relaxation from the LE state to ICT state undergoes cyanobiphenyl planarization processes and solvent reorganization. Furthermore, it is also found that the ICT state in high polar acetonitrile (ACN) leads to the faster formation of a triplet state with a higher yield than that in low polar toluene (TOL). According to the golden rule of radiationless transitions, it is found that the less planar ICT state geometry, larger spin-orbit coupling, and the smaller energy gap between high singlet and triplet states of the ICT state in ACN lead to the faster formation of the triplet state with a higher quantum yield. These results herein provide a guidance to understand the relationship between the solvent polarity-modulated LE/ICT state and the formation of triplet states.